1. Field of the Invention
The present disclosure relates to vibration control, and more particularly, to vibration control for lifting body carrying external cargo such as helicopters.
2. Description of Related Art
Aircraft commonly include structural elements that shift the natural frequency of the airframe away from the forcing frequency of vibration sources, typically by selectively stiffening portions of the airframe, such that vibration remains below predetermined levels to keep associated crew fatigue and airframe fatigue within manageable levels. The damping effect of such structural elements can change when the forcing frequency changes, such as when the aircraft operating regime changes, or when the natural frequency of the aircraft itself changes, such as when the aircraft weight and/or balance changes. For example, externally loaded cargo can cooperate with the aircraft to form a mass-spring-mass system having a natural frequency (and harmonics) that is different from the natural frequencies of the aircraft. In certain circumstances, the vibratory response of an aircraft carrying external cargo can be similar enough to a given aircraft forcing function that specialized rigging can be required to prevent excessive oscillation of the external load and/or aircraft structure. External cargo carrying operations can therefore require careful planning and execution, and in some circumstances, specialized rigging that is configured for a particular external load.
Such conventional methods and systems have generally been considered satisfactory for their intended purpose. However, there is still a need in the art for improved systems and methods for controlling aircraft vibratory response when carrying external cargo. The present disclosure provides a solution for this need.